Endless yarn tensioning strip and method for producing the same

ABSTRACT

An endless yarn braking strip for a yarn tensioning device consists of precipitation hardened stainless steel (S). The endless yarn braking strip is produced by first manufacturing an endless blank from a sheet of precipitation hardening steel, shaping the blank into a truncated cone in its endless form, and then hardening the strip by precipitation hardening.

[0001] The invention relates to an endless yarn braking strip accordingto the preamble part of claim 1 and to a method for manufacturing anendless yarn braking strip according to the preamble part of claim 2.

[0002] In a yarn brake comprising a frustoconical, endless braking stripthe inner surface of the braking strip is pressed coaxially against ayarn withdrawal surface of e.g. a storage body, the yarn withdrawalsurface in most cases being rounded. In this way a contact area iscreated between the inner surface of the braking band and the yarnwithdrawal surface which contact area is used as a braking zone. Theyarn is stored on the storage body in the form of windings and iswithdrawn from the windings overhead of the storage body and below thebraking strip. The yarn passes the contact area between the braking bandand the withdrawal rim in order to be braked, or to create asubstantially uniform yarn tension, respectively. By the withdrawal ofthe windings an orbiting rotation of the withdrawn yarn is generatedsimilar to the motion of the hand of a clock. The braking stripundergoes a deformation following the orbiting motion. The yarn rubs atthe braking strip. The storage body and the braking strip are maintainedsubstantially stationarily. For this reason the braking strip needs tohave flexibility, good spring property and high wear resistance againstthe friction load of the respective yarn material and against thefriction load at the withdrawal surface defining the braking zonetogether with the braking strip, which withdrawal surface in most casesis metallic.

[0003] A frustoconical endless braking strip is known from WO 98/23520.The braking strip consists of a metal or a metal alloy. In this caseberyllium-copper is mentioned as an example. Beryllium-copper is amaterial which needs extreme care during machining. In case of certainoperation conditions or for certain yarn qualities the braking strip,however, may become worn out relatively rapidly and markedly.

[0004] U.S. Pat. No. 5,546,994 discloses a braking strip consisting of ametal sheet with a thickness of 0.1 mm or less and which is produced bystamping and deep drawing. The braking strip is described as beingflexible.

[0005] U.S. Pat. No. 5,678,779 discloses an endless braking strip ofthis kind consisting of a metal alloy.

[0006] U.S. Pat. No. 5,409,043 discloses such a braking strip which isprovided in the form of a very thin metallic layer at the inner side ofan outer Kevlar carrier cone. The braking strip, alternatively, mayconsist of a steel sheet of a thickness of 0.05 to 0.1 mm. The activesurface of the strip may be chromium plated or nickel plated,respectively.

[0007] The braking strip has to be endless, springy, smooth despite thesmall thickness and wearproof at its active surface for a properperformance. The spring property and the wearproofness could be achievedby hardened conventional steel types. However, hardened conventionalsteel normally cannot be formed anymore. On the other hand, hardening ofprior deformed conventional steel hardly could be carried out until now.Hardening of conventional steel types after the forming process couldresult in form deficiencies of the frustocone in case of such thin wallthicknesses (maximum a few tenths of a millimetre) which formdeficiencies destroy the necessary evenness of the braking strip at theactive surface and which do not allow to achieve a homogeneousdeformation performance of the braking strip in operation. For thosereasons such yarn braking strips until now were produced from othermetallic materials.

[0008] A flexible yarn guiding sleeve known from U.S. Pat. No. 2,218,976A consists of steel. For improving the wear resistance only a free endsection of the yarn guiding sleeve is hardened by a heat treatment withsubsequent quenching.

[0009] The article W. T. LANKFORD, Jr. & AL.: “The Making, Shaping andTreating of Steel” 1985, HERBICK & HELD, PITTSBURGH, Pa. XPOO2193211,pages 1335, 1339, 1344, 1345 discusses the compositions of steels havinghigh amounts of Cr-Ni of the class “Precipitation Hardening StainlessSteel” as well as method steps carried out during precipitationhardening of those steel classes.

[0010] The section Beitz, K: H: Grote, Dubbel: “Taschenbuch für denMaschinenbau”, 19 Edition, Springer Verlag, Berlin, 1997, ISBN3-540-62467-8, E37-E39, discloses steel hardening processes fordifferent steel classes, the consequences of the hardening processes andseveral application examples for the hardened steels.

[0011] It is an object of the invention to provide a yarn braking stripof the kind as mentioned at the beginning as well as a method formanufacturing of such a yarn braking strip which allow to fulfil therequirements of wearproofness, uniform surface smoothness, uniformspring property and industrial large series production in a costeffective way.

[0012] The object can be achieved by the features of claim 1 and interms of the method by the features of claim 2.

[0013] Precipitation hardened stainless steel containing a high Cr-Niamount of the class “Precipitation Hardening Stainless Steel” as isnormally used for constructing aircrafts unexpectedly precisely fulfilsthe requirements which are valid for a braking strip of a yarn brake.That is, the steel can be formed comfortably prior to the hardeningprocess in order to form the frustoconical shape of the braking stripfrom a flat blank without form deficiencies and then can be hardenedsuch that it has the required springiness, the smooth surface and mainlythe wear resistancy at the active surface. Precipitation hardeningstainless steel is available in the desired thicknesses for fair costs,and can be simply machined mechanically and hardened comfortably.

[0014] In terms of the method the deformability of the precipitationhardenable stainless steel is used in order to form first thefrustoconical shape of the braking strip from the flat blank. Then thegood hardening property is used in view to high springiness and goodwearproofness at the active surface. In this case it astonishingly hasbeen found out that the hardening process can be carried out despite thethin wall thickness such that a smooth surface results and such that noform deficiencies of the frustocone of the yarn braking strip occur.

[0015] The flat blank, expediently, is formed by stamping. This allowsto achieve a high production rate with sufficient preciseness. The colddeformation into the form of the frustocone coat is expediently carriedout by deep drawing in a tool. The hardening process is carried out inthree steps, namely in an austenite conditioning step, a subsequentaustenite-martensite-transforming step and finally in an ultimateprecipitation hardening step.

[0016] During the austenite conditioning step the cold deformed blank isheated to about 955° C. and is then maintained about ten minutes at thistemperature. Thereafter the blank is cooled down in ambient air toambient temperature. Before an hour has expired the cooled down, colddeformed blank is cooled down to about −73° C. and is maintained at thistemperature for eight hours. Then the blank is allowed to warm inambient air again to ambient temperature (transformation step).Ultimately the blank is heated to about 510° C., is maintained at thistemperature for about 90 minutes, and finally is cooled down in ambientair to ambient temperature. By this the precipitation hardening isterminated. The braking strip then can undergo further conventionalmachining steps or may be directly integrated into the yarn brake,respectively.

[0017] In view to precise dimensions and homogeneous properties of thebraking strip, it may be expedient, to stamp the blank as a flat annulusover-dimensioned in radial direction and to cut the blank only after thecold deformation and prior to the hardening process to the targetdimensions.

[0018] The material provided in radial direction in the over-dimensionsis expedient to compensate for material displacements occurring duringthe stretching process accompanying the cold deformation. As a result ofthe subsequent cutting then uniform properties will be achieved in theyarn braking strip up to the final cutting edges.

[0019] Alternatively, the final cutting even may be carried out afterthe hardening process.

[0020] During the cold deformation, expediently, a uniform wallthickness is set between 0.01 mm to 0.5 mm. A thickness range of about0.05 mm to about 0.3 mm is of particular advantage for such yarn brakingstrips made from this precipitation hardened steel.

[0021] In order to avoid corrosion during storing and transportation orthe like which corrosion might disturb during the machining of thesteel, and to achieve a better machinability, the blank ought to bestamped from an annealed sheet metal. The term “annealed” is to beunderstood as a corrosion protection measure already employed by thesheet metal producer. This may, e.g., comprise a state which is called“Mill Annealed”, i.e., a state achieved by a treatment called “SolutionHeat Treated And Rapid Cooled”.

[0022] The invention will be explained with the help of the drawing. Inthe drawing is:

[0023]FIG. 1 a perspective view of an endless braking strip for a yarnbrake,

[0024]FIG. 2 the step of manufacturing a flat blank, in a verticalsection,

[0025]FIG. 3 schematically the cold deformation of the flat blank ofFIG. 2 into the shape of a frustocone, including a subsequent cutting ofthe blank into the final dimensions, and

[0026]FIG. 4 a vertical section of the braking strip, includingindications to the carried out annealing or hardening steps.

[0027] An endless braking strip B has in FIG. 1 the shape of afrustocone coat with a smaller diameter di and a larger diameter da, aheight h in the direction of the cone axis X, a strip width b in thedirection of the generatrice, and a wall thickness y. The wall thicknessy lies between about 0.01 mm and 0.5 mm and is uniform within the entirebraking strip B. The braking strip B consists of a precipitationhardened stainless steel S (precipitation

[0028] An endless braking strip B has in FIG. 1 the shape of afrustocone coat with a smaller diameter di and a larger diameter da, aheight h in the direction of the cone axis X, a strip width b in thedirection of the generatrice, and a wall thickness y. The wall thicknessy lies between about 0.01 mm and 0.5 mm and is uniform within the entirebraking strip B. The braking strip B consists of a precipitationhardened stainless steel S of the class “Precipitation HardeningStainless Steel”. A good operation performance can be achieved with thewall thickness of 0.08 mm, e.g. in case of a yarn braking strip havingan outer diameter of about 110 mm, and inner diameter of about 85 mm,and a cone apex angle between about 90° and 120°.

[0029] The inner surface of the braking strip B is the active brakingsurface which has to be smooth and wear resistant. Furthermore, thestrip has to be unstretchable but must be deformable in radial directionor must be springy in radial direction.

[0030] For manufacturing the braking strip B of FIG. 1 the followingsteps are carried out:

[0031] According to FIG. 2 first an annular flat blank Z is formed froma flat sheet metal M, e.g. by stamping. The inner diameter of theannulus is smaller than the target inner diameter di of the brakingstrip. The outer diameter of the blank is larger than the target outerdiameter da of the braking strip B.

[0032] According to FIG. 3 the flat blank Z is brought to the shape ofthe frustocone or of a frustoconical intermediate product Z1 in a toolW, e.g. by deep drawing. Since unavoidably material displacements occurduring a deep drawing process the blank Z is oversized in radialdirection. The over dimensions then will be used during the deformationstep of FIG. 3 in order to allow a cold flow of the material. After thedeformation steps the blank is cut such that the target diameters di andda result.

[0033] The blank Z1 now having already the final dimensions then istreated as follows:

[0034] During an austenite conditioning step I the blank Z1 first isheated to e.g. 955° C. and is maintained for about ten minutes at thistemperature. Then the blank is allowed to cool down in ambient air againto ambient temperature RT.

[0035] In a second austenite-martensite-transforming step II whichstarts within one hour after the first step I the blank Z1 is cooleddown to about −73° C. and is then maintained at this temperature forabout eight hours before the blank is allowed to again warm in ambientair to ambient temperature RT.

[0036] In a subsequent precipitation hardening step III blank Z1 againis heated, optionally to about 510° C., is maintained for ninety minutesat this temperature, and then is allowed to cool down in ambient air toambient temperature RT. Then the braking strip B consisting of theprecipitation hardened stainless steel S is manufactured.

[0037] A subsequent machining is not necessary, however, mayoccasionally be carried out.

[0038] Among other ingredients precipitation hardenable stainless steelof the class “Precipitation Hardening Stainless Steel” contains chromiumand nickel as main alloy ingredients. This steel type per se is intendedfor manufacturing springs, clips, frame structures in aircrafts andpressure tanks. For such applications the wear resistance of this steelis of secondary importance. According to the invention to the contrary,mainly the wear resistance of the precipitation hardenable steel is usedas an extremely favourable side effect for braking yarns.

1. Endless braking strip (B) for a yarn brake, the braking strip havingthe shape of a frustocone coat produced by cold deformation of a flatblank (Z) made from thin metal sheet (M), characterised in that thebraking strip (B) consists of precipitation hardened stainless steel (S)of the class “Precipitation Hardening Stainless Steel” containingchromium and nickel as main alloy ingredients.
 2. Method formanufacturing an endless braking strip for a yarn brake according toclaim 1, wherein the method comprises the following sequential methodsteps, cutting the endless flat blank (Z) from the sheet metal (M), coldforming of the blank (Z) into the shape of a frustocone, precipitationhardening of the frustoconical blank (Z1).
 3. Method as in claim 2,characterised by stamping the blank (Z), deep drawing the stamped blank(Z) into the frustoconical shape, and precipitation hardening of thedeep drawn blank (Z1) an in an austenite conditioning step (I), asubsequent austenite-martensite-transforming step (II) and a finalprecipitation hardening step (III) .
 4. Method as in claim 2,characterised by heating the deep drawn blank (Z1) to slightly below1000° C., preferably to 955° C., and maintaining this temperature forabout ten minutes, and cooling down in ambient air to ambienttemperature (RT) during the austenite conditioning step (I), cooling theblank (Z1) subsequently within one hour to about −73° C., maintainingthis cooling temperature over about eight hours during theaustenite-martensite-transforming step (II), and warming in ambient airto ambient temperature, and heating the blank to somewhat above 500° C.,preferably 510° C., maintaining this temperature over about ninetyminutes, and cooling down in ambient air to ambient temperature (RT)during the precipitation hardening step (III).
 5. Method as in claim 2,characterised by stamping the blank (Z) as a flat annulus with overdimensions in radial direction and cutting the blank in radial directionto the target dimensions (di, da) first after the deep drawing and priorto the precipitation hardening process.
 6. Method as in claim 2,characterised by stamping the blank (Z) as a flat annulus with overdimensions in radial direction and cutting the blank in radial directionto the target dimensions (di, da) first after the precipitationhardening step.
 7. Method as in claim 2, characterised by deep drawingthe blank into a uniform wall thickness within a range (y) between 0.01mm and 0.5 mm, preferably between 0.05 mm and 0.3 mm.
 8. Method as in atleast one of claims 2 to 7, characterised by annealing the sheet metal(M) prior to stamping the blank (Z).